1. Field of the Invention
The present invention generally relates to communication systems, and more specifically to maintaining synchronization in a communication system.
2. Background
In conventional cable modem systems, for example, a coaxial or hybrid fiber-coaxial (HFC) network provides a point-to-multipoint topology to support bidirectional data communication between a cable modem termination system (CMTS) at the cable headend and multiple cable modems at the customer premises. The communication path from the CMTS to the cable modems is typically referred to as the downstream, while the communication path from the cable modems to the CMTS is typically referred to as the upstream.
DOCSIS (Data Over Cable Service Interface Specification) refers to a group of specifications published by CableLabs® that define industry standards for cable headend and cable modem equipment. In part, DOCSIS sets forth requirements and objectives for various aspects of cable modem systems including operations support systems, management, data interfaces, as well as network layer, data link layer, and physical layer transport for data over cable systems. The current version of the DOCSIS specification is version 2.0, and includes the DOCSIS Radio Frequency Interface (RFI) Specification SP-RFIv2.0-I03-021218 (hereinafter “DOCSIS RFI Specification”), the entirety of which is incorporated by reference herein.
DOCSIS supports the ITU-T J.83 B (hereinafter “Annex B”) standard for downstream physical (PHY) layer transmissions from the CMTS to cable modems. One difference between DOCSIS 2.0 and earlier versions of the standard is that DOCSIS 2.0 supports two different burst type formats for upstream PHY layer transmissions from the cable modems to the CMTS. In particular, as specified by the DOCSIS RFI Specification, the upstream physical media dependent (PMD) sublayer can use either an FDMA/TDMA burst type format, often referred to as the “TDMA mode,” or an FDMA/TDMA/S-CDMA burst type format, often referred to as the “S-CDMA mode.” The CMTS configures a cable modem to use either the S-CDMA mode or the TDMA mode through the transmission of DOCSIS media access control (MAC) messages.
The S-CDMA mode requires more precise synchronization than the TDMA mode. For example, a cable modem that uses S-CDMA generally requires synchronization within one or two nanoseconds to prevent interference with other S-CDMA modems transmitting information on the same upstream channel. On the other hand, a cable modem that uses TDMA typically requires synchronization within 250 nanoseconds. The precision of the synchronization required for S-CDMA is therefore approximately 100 or 200 times greater than that required for TDMA.
Regardless whether a cable modem is configured to use S-CDMA or TDMA, the timing of the upstream signal is generally based on timing information included in the downstream signal. Cable modems using earlier versions of the DOCSIS standard, such as DOCSIS 1.0 or DOCSIS 1.1, use timestamps provided in the downstream signal to synchronize the upstream and downstream signals. However, the heightened synchronization requirements of S-CDMA render timestamps insufficient for many cable modems using the DOCSIS 2.0 standard. Timestamps often do not occur with sufficient frequency to provide the additional 100 or 200 times greater precision in synchronization required in the S-CDMA mode.
One proposed solution is for a CMTS transmitter to provide a symbol clock, or baud clock, within the downstream signal, which is then used by the cable modem to synchronize its upstream transmissions. However, if the transmitter fails or if the downstream signal is lost, the cable modem loses synchronization and all previously-received timing information is typically discarded. As a result, the cable modem typically resets and is required to execute a time-consuming re-ranging protocol with the CMTS in order to re-acquire synchronization.
What is needed, then, is a method and apparatus for maintaining synchronization in a communication system that addresses one or more of the aforementioned shortcomings of conventional communication system equipment and methods.